In an internal combustion engine, engine valve actuation is required in order to produce positive power, and may also be used to produce engine braking and/or exhaust gas recirculation (EGR). During positive power, one or more intake valves may be opened to admit air into a cylinder for combustion during the intake stroke of the piston. One or more exhaust valves may be opened to allow combustion gases to escape from the cylinder during the exhaust stroke of the piston.
One or more exhaust valves may also be selectively opened to convert, at least temporarily, the engine into an air compressor for engine braking operation. This air compressor effect may be accomplished by either cracking open one or more exhaust valves near piston top dead center (TDC) position for compression-release type braking, or by maintaining one or more exhaust valves in a cracked open position during much or all of the piston motion, for bleeder type braking. In either of these methods, the engine may develop a retarding force that may be used to help slow a vehicle down. This braking force may provide the operator with increased control over the vehicle, and may also substantially reduce the wear on the service brakes. Engine braking has been long known and is disclosed in Cummins, U.S. Pat. No. 3,220,392 (November 1965), which is hereby incorporated by reference.
The braking power of a compression-release type engine brake may be increased by selectively actuating the exhaust valves to carry out brake gas recirculation in combination with compression release braking. Brake gas recirculation (BGR) denotes the process of opening an exhaust or auxiliary valve on the intake or expansion stroke of the piston and/or opening an intake or auxiliary valve during the exhaust or compression stroke of the engine. During engine braking, the introduction of exhaust gases from the exhaust manifold into the cylinder may increase the total gas mass in the cylinder at the time of the compression release event. This increased gas mass in the engine cylinder may increase the braking effect realized by the compression-release event.
An example of a lost motion system and method used to obtain retarding and brake gas recirculation is provided by Gobert, U.S. Pat. No. 5,146,890 (Sep. 15, 1992) which discloses a method of conducting brake gas recirculation by placing the cylinder in communication with the exhaust system during the first part of the compression stroke and optionally also during the latter part of the intake stroke, and which is hereby incorporated by reference. Gobert uses a lost motion system to enable and disable retarding and brake gas recirculation, but such system is not variable within an engine cycle, i.e., this system does not provide variable valve actuation (VVA).
Intake, exhaust, and/or auxiliary valves may also be actuated to provide exhaust gas recirculation (EGR) for improved engine performance during positive power operation. Actuating the exhaust valve during positive power to provide EGR may cause exhaust gas in the exhaust manifold to flow back into the cylinder and/or exhaust gas in the cylinder to flow back into the intake manifold. The recirculation of the exhaust gases may lower the combustion temperature and reduce NOx emissions. An example of the use of EGR to reduce NOx emissions during positive power operation of an engine is disclosed in Israel, U.S. Pat. No. 6,170,474 (Jan. 9, 2001), which is hereby incorporated by reference.
In many internal combustion engines, the intake and exhaust valves may be actuated by fixed profile cams, and more specifically, by one or more fixed lobes that are an integral part of each cam. For example, an intake cam profile may include an additional lobe for EGR/BGR prior to the main intake lobe, and/or an exhaust cam profile may include an additional lobe for EGR/BGR after the main exhaust lobe. Other auxiliary lobes may be included on the cam to provide cylinder charging events, compression-release events, or bleeder braking events. The fixed profile cams will produce fixed valve events in terms of timing and lift unless a specialized system is included in the valve train to provide variable valve actuation.
Benefits such as increased performance, improved fuel economy, lower emissions, increased braking power, and/or better vehicle drivability may be obtained if the intake and exhaust valve timing and/or lift can be varied using a variable valve actuation system. It may be particularly beneficial to adjust valve timing and/or lift to improve performance based on changes to various engine operating conditions, such as different engine speeds, loads, and engine component temperatures and pressures.
One method of adjusting valve timing and lift, given a fixed cam profile, has been to provide variable valve actuation (VVA) by incorporating a lost motion device in the valve train between the valve and the cam. Lost motion is the term applied to a class of technical solutions for modifying the valve motion proscribed by a cam profile with a variable length mechanical, hydraulic, or other linkage assembly. In a lost motion system, a cam lobe may provide the maximum motion (longest dwell and greatest lift) needed over a full range of engine operating conditions. A variable length system may then be included in the valve train intermediate of the valve to be opened and the cam providing the maximum valve actuation motion, to subtract or lose part or all of the motion imparted by the cam to the valve. The lost motion VVA system may be used to selectively cancel or activate any or all combinations of valve lifts possible from the assortment of lobes provided on the intake and exhaust cams.
Engine benefits from lost motion VVA systems can be achieved by creating complex cam profiles with extra lobes or bumps to provide auxiliary valve lifts in addition to the conventional main intake and exhaust events. Many unique modes of engine valve actuation may be produced by a VVA system that includes multi-lobed cams. As a result, significant improvements may be made to both positive power and engine braking operation of the engine. Examples of VVA systems are disclosed in Vorih et al., U.S. Pat. No. 6,510,824 (Jan. 28, 2003), entitled “Variable Lost Motion Valve Actuation and Method;” and Vanderpoel et al., U.S. patent application Pub. No. US 2003/0221663 A1 (Dec. 4, 2003) entitled “Compact Lost Motion System for Variable Valve Actuation,” both of which are incorporated herein by reference.
It may also be desirable to increase the exhaust back pressure in the exhaust manifold during engine braking, and in particular compression-release braking. During compression-release engine braking, a large force may be needed to open the exhaust valve against the relatively high pressure that occurs in the engine cylinder near piston top dead center position. Increased exhaust back pressure may increase the pressure on the back side of the valve which may counter the pressure exerted by the gases in the cylinder and thus reduce the loading on the mechanism used to open the exhaust valve for compression-release events. Increased exhaust back pressure may also increase the pressure in the engine cylinder during the piston's compression stroke and thereby increase the braking power that the piston exerts on the crankshaft.
Increasing the pressure of gases in the exhaust manifold may be accomplished by restricting the flow of gases through the exhaust manifold. Exhaust manifold restriction may be accomplished through the use of any structure that restricts all or partially all of the flow of exhaust gases through the exhaust manifold. The exhaust restrictor may be in the form of an exhaust brake, a turbocharger, a variable geometry turbocharger, a variable geometry turbocharger with a variable nozzle turbine, and/or any other device which may limit the flow of exhaust gases through the engine and exhaust system.
Exhaust brakes generally provide restriction by closing off all or part of the exhaust manifold, thereby preventing the exhaust gases from escaping. This restriction of the exhaust gases may provide a braking effect on the engine by providing back pressure when each cylinder is on the exhaust stroke. For example, Meneely, U.S. Pat. No. 4,848,289 (Jul. 18, 1989); Schaefer, U.S. Pat. No. 6,109,027 (Aug. 29, 2000); Israel, U.S. Pat. No. 6,170,474 (Jan. 9, 2001); Kinerson et al., U.S. Pat. No. 6,179,096 (Jan. 30, 2001); and Anderson et al., U.S. patent application Pub. No. US 2003/0019470 (Jan. 30, 2003) disclose exhaust brakes for use in retarding engines.
Turbochargers may similarly restrict exhaust gas flow from the exhaust manifold. Turbochargers often use the flow of high pressure exhaust gases from the exhaust manifold to power a turbine. A variable geometry turbocharger (VGT) may alter the amount of the high pressure exhaust gases that it utilizes to drive a turbine. For example, Arnold et al., U.S. Pat. No. 6,269,642 (Aug. 7, 2001) discloses a variable geometry turbocharger capable of modifying the angle and the length of the vanes in a turbine to vary the amount of exhaust gas restriction. An example of the use of a variable geometry turbocharger in connection with engine braking is disclosed in Faletti et al., U.S. Pat. No. 5,813,231, which is hereby incorporated by reference.